1. Field of the Disclosure
Exemplary embodiments of the present disclosure relate to a fixing device and an image forming apparatus including the fixing device, and more specifically, to a fixing device that applies heat and pressure to a recording medium at a nip formed between a fixing member and a pressing member to fix an image on the recording medium, and an image forming apparatus including the fixing device.
2. Description of the Background
As one type of image forming apparatus, electrophotographic image forming apparatuses are widely known. In an image formation process executed by an electrophotographic image forming apparatus, for example, a charger uniformly charges a surface of an image carrier (e.g., photoconductor drum); an optical writing unit directs a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is either directly transferred from the image carrier onto a recording medium or indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device may be either a belt-type fixing device or a film-type fixing device. FIG. 1 shows a schematic configuration of a conventional belt-type fixing device. In FIG. 1, the belt-type fixing device includes a heating roller 202, a fixing roller 203, a fixing belt 204, and a pressing roller 205. The heating roller 202 includes a heater 201. The fixing roller 203 includes a rubber layer on its surface. The fixing belt 204 is stretched between and wound around the heating roller 202 and the fixing roller 203. The pressing roller 205 presses against the fixing roller 203 via the fixing belt 204 to form a fixing nip N through which a recording medium passes.
More specifically, to transfer a toner image onto a sheet of recording medium P, the recording medium P is conveyed to the fixing nip N between the fixing belt 204 and the pressing roller 205. When the recording medium P passes through the fixing nip N, heat and pressure are applied to the toner image on the recording medium P to fix the toner image on the recording medium P.
By contrast, FIG. 2 shows a schematic configuration of a conventional film-type fixing device. As described in JP-H04-044075-A, typically, a ceramic heater 211 and a pressing roller 212 together sandwich a heat-resistant film 213, which is the functional equivalent of the fixing belt described above, to form the fixing nip N. A recording sheet is fed to the fixing nip N between the heat-resistant film 213 and the pressing roller 212. Then, the recording sheet is sandwiched by the heat-resistant film 213 and the pressing roller 212 to be conveyed together with the heat-resistant film 213.
Further, the film-type fixing device may be an on-demand type fixing device, including a ceramic heater and a film member of low heat capacity. In an image forming apparatus including the fixing device, the ceramic heater is turned on only during image formation to generate heat at a certain fixing temperature to shorten a waiting time required to reach to a state ready for image formation from activation of the image forming apparatus, thereby reducing power consumption in a standby mode.
Further, a conventional pressing-belt-type fixing device like that described in JP-H08-262903-A includes a thermal fixing roller, an endless belt, and a pressing pad. The thermal fixing roller is rotatable and has an elastically deformable surface. The endless belt travels in contact with the thermal fixing roller. The pressing pad is fixedly mounted inside a loop formed by the endless belt and presses the endless belt against the thermal fixing roller to form a belt nip between the endless belt and the thermal fixing roller through which the recording medium passes.
According to the pressing-belt-type fixing device described above, pressure of the pressing pad against the endless belt elastically deforms the surface of the thermal fixing roller and enlarges a contact area of the thermal fixing roller and the recording medium to enhance heat conduction efficiency, reduce energy consumption, and achieve a more compact design.
However, for example, in the above-described film-type fixing device described in JP-H04-044075-A, there is room for improvement in durability and temperature stability of the fixing belt.
For example, the fixing belt is made of heat-resistant film and is abrasion-resistant. However, since the fixing belt slides over the ceramic heater as the fixing belt rotates, the fixing belt tends to get worn out when driven for an extended period of time. Accordingly, rotation of the fixing belt may become unsteady and/or the driving torque required by the fixing device may increase, neither of which is desirable. Consequently, the recording medium may slip on the fixing belt, causing displacement of a resultant image. Alternatively, a driving gear may be subjected to increased stress, causing damage to the gear.
Further, in the film-type fixing device, the fixing belt is heated locally, that is, only at the fixing nip. As a result, the temperature of the fixing belt is at its lowest when the fixing belt in rotation returns to an entrance of the fixing nip, causing faulty fixing, particularly at high-speed rotation.
To reduce the friction between the fixing belt and the ceramic heater or other stationary members, for example, JP-H08-262903-A describes a fixing device using a fiberglass sheet (PTFE impregnated glass cloth) impregnated with polytetrafluoroethylene (PTFE) as a low-friction sheet (a sheet-shaped slide member) as a surface layer of the pressing pad.
However, in the above-described pressing-belt-type fixing device, a large heat capacity of the fixing roller may increase the time required for raising the temperature of the fixing roller to the required level, thereby extending the warm-up time.
Hence, techniques like those described in JP-2007-334205-A and P-2008-158482-A are proposed to meet such challenges. However, although successful at shortening the warm-up time, such fixing devices are susceptible to new problems caused by a difference in temperature (a temperature differential) between the fixing member and the pressing member.
Specifically, in such fixing devices, the temperature of the fixing member rises more quickly than the temperature of the pressing member does. Consequently, a temperature differential arises between opposed front and back faces of the recording sheet passing through the fixing nip may occur, causing curling of the recording sheet. In other words, a difference in the moisture content evaporated from the front and back faces of the recording sheet may cause curling of the recording sheet.
Further, the inventors of the present disclosure have found that such curling of the recording sheet might also be caused by the shape (e.g., curvature) of the fixing nip as well as the above-described temperature differential between the front and back faces of the recording sheet.